It is known that electrical equipment and devices, such as high voltage DC transformers, are usually equipped with bushings, which are suitable to carry current at high potential through a grounded barrier, e.g. a transformer tank or a wall. Conventional bushings are constituted by an insulator made of ceramic or composite material, which is provided with sheds and is generally hollow, and on the inside can the voltage grading be performed with or without a condenser body through which the electrical conductor passes, allowing to connect the inside of the device on which the bushing is fitted to the outside.
An example of a prior art bushing adapted for use with a high voltage do transformer will now be described with reference to FIGS. 1-3, wherein FIG. 1 shows the overall structure of the bushing, generally referenced 1, FIG. 2 is an overall cross-sectional view of the bushing mounted to a transformer housing, and FIG. 3 is a detailed sectional view of the area enclosed by the dashed line in FIG. 2.
A high voltage conductor 10 runs through the center of a hollow bushing insulator 12 that forms a housing around the high voltage conductor. A condenser core 14 is provided inside the insulator housing for voltage grading which is build up around the high voltage conductor 10. A flange 16 is provided to connect the housing of the bushing to ground through a tank assembly housing, schematically shown as 18 in FIG. 2. A ground potential grading shield (not shown) may be mounted to the flange.
The bottom end portion of the high voltage conductor 10 forms a bottom contact 20, which is arranged to be connected to the internal components of the transformer. An upper outer terminal 24 is provided at the end of the bushing opposite the bottom contact end in order to electrically connect the transformer device to external sources.
In high voltage DC applications, material resistivity becomes important. Materials surrounding the condenser core may become more essential for the voltage distribution than the condenser core itself. Oil with a relatively low resistivity and short time constant compared to the composite materials may become the most important part for voltage grading.
Turning now to FIG. 3, it is seen that an annular or cylindrical oil duct 26 having a constant width in a radial direction is provided between the condenser core 14 and a composite barrier 28. The oil duct has tapering end portions which follow the outer contour of the condenser core. The function of the oil duct is to act mainly as a flexible dielectric interface between the condenser core and the composite barrier.
The space 30 outside the composite barrier 28 is filled with insulating gas, such as SF6, to provide electrical isolation between the barrier and the hollow bushing insulator 12.
In a DC bushing, a voltage potential distribution is built up mainly in a radial direction from the grounded flange 16 and inwards to the high voltage conductor 10. However, along the tapering portions of the oil duct and the composite barrier, the voltage potential distribution also has an axial component. In the oil duct, this distribution is governed by the resistance of the oil in an axial direction. This resistance can be expressed as follows:R/A, wherein R is the resistivity of the oil and A is the total cross-sectional area of the oil duct. The area of the oil duct can be expressed as follows, using the parameters shown in FIG. 4:A=π(ΔR+2r1Δr)wherein r2 is the outer radius of the oil duct, r1 is the inner radius of the oil duct, and Δr is r2−r1.
In the prior art bushing shown in FIG. 3, the width of the oil duct is constant, i.e., Δr is a constant along the length of the bushing. In the tapering outer portions of the bushing, the outer and inner radiuses decrease in the direction of the end portions. This in turn means that the resistance per axial length unit increases in the direction of the end portions since the total area of the oil ducts decreases, given constant Δr.